Honeycomb structures are in use, for example, in a filter for capturing fine particles present in an exhaust gas emitted from an internal combustion engine, a boiler or the like, particularly diesel fine particles, as well as in a carrier for exhaust gas purification catalyst.
Honeycomb structures used, for example, as a filter generally have, as shown in FIGS. 2(a) and 2(b), a large number of through-holes 3 divided by partition walls 2 and extending in the X-axis direction, wherein each through-hole 3 is plugged at either end of the hole and adjacent through-holes 3 are plugged alternately at each end of the structure so that each end of the structure looks a checkerboard pattern. In a honeycomb structure having such a constitution, a to-be-treated fluid, for example, enters, at one end 42 of the structure, those through-holes 3 which are not blocked at the end 42 but are blocked at other end 44, passes through porous partition walls 2, and is discharged from adjacent through-holes 3 which are blocked at the one end 42 but not blocked at the other end 44. At this time, the partition walls 2 function as a filter and, for example, a soot emitted from a diesel engine is captured by the partition walls and deposits thereon. In the honeycomb structure used for such a purpose, the sharp temperature change of exhaust gas and the local heating makes non-uniform the temperature distribution inside the honeycomb structure and there have been problems such as thermal stress generation in honeycomb structure, crack formation and the like. When the honeycomb structure is used particularly as a filter for capturing a particulate substance in an exhaust gas emitted from a diesel engine (this filter is hereinafter referred to as DPF), it is necessary to burn the fine carbon particles accumulated on the filter to remove the particles and regenerate the filter; in this case, high temperatures are generated locally in the filter, this non-uniform temperature distribution during regeneration produces a big thermal stress, and cracks have tended to arise.
Hence, there were proposed processes for producing a honeycomb structure by bonding a plurality of individual honeycomb segments using an adhesive. In, for example, U.S. Pat. No. 4,335,783 is disclosed a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. Also in JP-B-61-51240 is proposed a heat shock-resistant rotary regenerating heat exchanging method which comprises forming, by extrusion, matrix segments of honeycomb structure made of a ceramic material, firing them, making smooth, by processing, the outer peripheral portions of the fired segments, coating the to-be-bonded areas of the resulting segments with a ceramic adhesive having, when fired, substantially the same mineral composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800° C., and firing the coated segments. Also in a SAE article 860008 of 1986 is disclosed a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement. Further in JP-A-8-28246 is disclosed a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles.
By thus adopting a segmentalized honeycomb structure, cracks formation caused by thermal stress can be prevented to some extent. However, if there is developed a honeycomb structure made of a material having an improved thermal conductivity, local appearance of high temperatures can be prevented and a practical thermal shock resistance can be achieved without reducing the number of segments or even without adopting a segmentalized honeycomb structure.
Meanwhile, with respect to the material of honeycomb structure, use of a Si—SiC type material in DPF is proposed in JP-B-8-13706. In the literature, it is disclosed that the material is superior in heat resistance, thermal shock resistance and oxidation resistance. However, the literature makes no mention on the thermal conductivity of the material.